Heretofore it has been known that the amplitude of line voltages often changes with time. Such changes of line voltage necessarily result in changes in the power input and output of devices operating off of the line voltage. However, certain devices require a relatively constant power output irrespective of changes in power input. By way of example only, air knives, used for transporting paper in copy machines, require that a constant level of vacuum be delivered by the vacuum motor and that the same not change over a course of time, regardless of changes on the power input line. While an air knife is only a single example of a device requiring the benefits of the instant invention, consistency of operation is desired in numerous devices and applications, all of which may benefit from the invention presented herein.
It is further desirable to assure constant performance of various devices at different frequencies of input voltage. It is known that voltage and frequencies vary from country to country. It is most desirable to provide a control circuit which assures proper efficient operation within a range of line frequencies to accommodate such difference.
In a vacuum device, such as an air knife, it is also known that the resultant vacuum is affected by altitude and, accordingly, it is desirable to provide a means for compensating for changes in altitude. Further, it is known that operation varies with temperature changes and, accordingly, it is desirable to provide a control circuit which renders motor operation impervious to such temperature changes.